Going to have a dedicated line installed- What should i know to ask for?

Lots of good posts already so I won't add any redundancy, just will comment that I ran a dedicated (also called a "home run") line to my dedicated listening room. The results were entirely satisfactory. Worth the $, IMO. YMMV

Tom

Sometimes, it's just a matter of luck. Our house was built in 1952, the original wiring was two-wire, w/o ground. The boxes are all metal, and are (from all I've seen) grounded separately from the circuits. It's dead quiet, zero hum.  In the twenty years we've been here, we've replaced the fuse boxes with a modern breaker panel, replaced the original meter with a 400A meter socket, re-did the service drop, replaced much of the old two-wire with modern Romex w/ground, as I've been able during remodels, etc. It was a fairly basic arrangement, done well (for the time), and wasn't ever botched up.My only caution to anyone would be to have the work professionally done, if you aren't certain what you're doing. Goes double, if you're in a multi-family dwelling.

Dedicated line is great. You could isolate the dedicated line completely by having your electrician install a isolation transformer. Square D makes them as well as many other manufacturers. You could even go so far as to place the transformer between your panel and subpanel (if you went wild and installed that too).
But a #12awg (2) wire plus ground, metal clad cable, or EMT raceway with wire to feed a high quality plug will work wonders for the sound quality. I think isolation transformers are the best if you live in a building with many other residents feeding off the same main service you are.

@jwolchak- I'm not sure that's true. For a residential application the iso transformer would have to be tied back to the main electrical system ground. (You cannot float the ground on the secondary, as I understand it, or ground to a different grounding point without violating Code, at least in the States). 
It will nonetheless isolate from certain things, but not everything. I'm currently using a large 10kVa isolation transformer from Controlled Power with a high K rating between the panel and subpanel for my system and it's great; the unit, which is large at 400+pounds, is installed in a weatherproof cabinet outside.
This is a more standard EI type, not a toroid. I'm a pretty big fan of iso transformers.
I've encountered different meanings for 'fully or completely isolated' in this context- some see a completely separate ground as essential but I don't think it is possible to do for a residence and stay within Code. (I'm not a Code expert or electrician, just used these things and dealt with their installation). Others here with a working knowledge of Code could amplify or correct any misstatement on my part. 
Welcome to the 'Gon.